Modify Markdown parser to inject classes automatically

[querkmachine]

Modifies the Markdown parser so that the appropriate class names are added to the elements on build time. This means we can use the styles directly, instead of using a prose component to 'translate' the classes onto raw elements.

Heavily based on markdown-it-govuk.

Changes

• Modifies markdown-it to:
  • add class names to headings, paragraphs, links, lists, code blocks, horizontal rules, blockquotes, tables and images
  • transform vulgar fractions into their unicode alternatives (e.g. 3/4 becomes ¾)
  • transform ~= into ≈ because I use that symbol an awful lot actually
  • transform >= and <= into ≥ and ≤
  • transform the letter x into a multiplication symbol if it has numbers on both sides (e.g. 1024x768 becomes 1024×768)
• Modifies various prosaic styles to make them work nicely without the need for dedicated prose styles (i.e. have a bottom margin please).
• Makes general visual updates for lots of prosaic components.
  • Slightly darkens the light-green colour so that it provides stronger contrast against white.
• Updates the test page and beeps' brand pages accordingly.
• Removes unnecessary instances of kimProse in templates.